1. Technical Field
The present invention relates to the correction of spinal columns. In particular, the present invention relates to a surgically implantable device for maintaining vertebral bodies of the spinal column in a desired relationship during healing of a portion of the spinal column.
2. Description of the Prior Art
It is known that spinal columns, particularly human spinal columns, are prone to deformation and degeneration. These conditions often require surgery to correct. During one type of surgical procedure, at least one of the intervertebral discs is removed from a damaged portion of the spinal column. Bone graft is prepared and is placed in the position from which the damaged disc has been removed. A device is connected with the vertebral bodies on opposite sides of the bone graft to maintain the vertebral bodies in a desired relationship and to prevent load from being transmitted through the bone graft. It is desirable to prevent the transmission of load through the bone graft immediately after surgery because the bone graft is not in a healed condition sufficient enough to transmit load therethrough.
U.S. Pat. No. 4,611,581 discloses such a device for connection with vertebral bodies. The device includes an elongate plate having a plurality of openings arranged along the longitudinal central axis of the plate. Fasteners are received in the openings to connect the plate with the vertebral bodies. The plate is relatively rigid and transmits load of the spinal column through the plate so the load is not transmitted through the bone graft.
However, the implantable device generally transmits load of the spinal column therethrough long after the bone graft has healed. In some instances, it is felt, that because the surgically implantable device transmits a relatively large portion of the load therethrough that the bone graft does not heal as quickly as if some load was transmitted through the bone graft. It is believed that bone graft heals more quickly if, as it heals, the load transmitted through the bone graft progressively increases.
A known external fixation device, such as disclosed in U.S. Pat. No. 1,789,060, is used on other bones in the body to reduce bone fractures and promote their healing. The known external fixation device has a portion extending outside of the body. The device can be adjusted to decrease the portion of the load transmitted through the device and concurrently increase the portion of the load transmitted through the healing bone. It has been found that such a procedure promotes better healing of the injured bone.
However, it is impractical to have a portion of the aforementioned surgically implanatable device connected to vertebral bodies of a spinal column extending outside of the body for adjustment during the course of healing of the bone graft. It is also impractical to perform surgery each time an adjustment is desired on the implanted device. Thus, it will be apparent that a need exists for a completely internal apparatus connectable with vertebral bodies of a spinal column and which decreases the amount of load transmitted therethrough as bone graft heals and thereby increases the load transmitted through the vertebral bodies and bone graft for promoting healing of the bone graft.